The present invention relates to a dot printer which prints characters with dot printing elements provided on printing hammers, as disclosed in U.S. Pat. No. 3,941,051, or with dot printing elements which are printing wires, as disclosed in U.S. Pat. No. 3,999,644.
The case where characters are printed with dot printing elements on printing hammers will be described. As shown in FIG. 1, dot printing element 2 secured in the upper end portions of printing hammers 1 are arranged in a single line parallel to a printing line. The lower end portions of the printing hammers 1 are secured to a hammer base 4 with fixing screws 3.
An example of a conventional dot printer is shown in FIG. 2. The printing hammers 1 are held in a hammer bank 5 through the hammer base 4. The hammer bank 5 is coupled to a drive motor 7 through a power conversion mechanism 6 including, for instance, a cam and a cam follower, so that the hammer bank 5 can be reciprocated along a printing line by the motor 7. An ink ribbon (not shown) and a printing sheet 8 are inserted between the dot printing elements 2 in the hammer bank 5 and a platen 11. The printing sheet 8 is fed by a tractor 9 which is driven by a sheet feeding motor 10.
As the hammer bank 5 is moved forwardly during reciprocation, dots in one horizontal line are printed by the dot printing elements 2 after which the printing sheet 8 is moved to the next dot printing line whereupon dots in that line are printed as the hammer bank 5 is moved backwardly, or returned. This operation is repeatedly carried out to print dots horizontally and vertically. That is, matrix characters are printed by reciprocating the hammer bank 5 parallel to a printing line and by stepping the printing sheet 8. When characters in one line have been printed, the printing sheet 8 is moved to the next printing line.
The printing speed of the dot printer of this type can be increased by employing a structure in which dot printing elements 2 are provided in the same number as there are positions and each dot printing element 2 prints a dot at the respective character column position. However, the provision of dot printing elements 2 and accordingly printing hammers 1 for each character column position involves the following drawbacks: If the printing hammers 1 are provided for each character column position, then it is necessary to provide an exciting device composed, for instance, of a permanent magnet and an electromagnetic coil for each character column position. This greatly increases the weights of the hammer bank 5 and the required capacity of the motor 7 making it difficult to reciprocate the hammer bank 5 at a high speed. As a consequence, it is impossible to increase the printing speed.
In order to overcome these drawbacks, another printer structure has been proposed in the art. In that printer, a dot printing element 2 is used to print dots at two or three character column positions so that the number of printing hammers 1 and exciting devices is decreased and accordingly the weight of the hammer bank 5 is decreased to thereby allow the hammer bank 5 to be reciprocated at a high speed and thereby increasing the printing speed.
However, that structure still involves the following drawbacks. If the hammer bank 5 is reciprocated at a high speed, then when the direction of movement of the hammer bank 5 is reversed, i.e. when the hammer bank 5 is returned, as the acceleration of the hammer bank 5 typically reaches 20 to 30 G, with a hammer bank 5 having a weight of 2 kg an inertial force of 40 to 60 kg is imposed. As a result, a high force is imparted to the printer itself in the direction of movement of the hammer bank 5 thus causing the printer to vibrate strongly.
In order to solve this problem, a technique has been proposed in which a counterweight is provided reciprocating in directions opposite to the directions of movement of the hammer bank 5 to cancel the aforementioned inertial force and thereby preventing vibration of the printer. However, that technique is also disadvantageous in that the counterweight is unnecessary to the basic function of the printer, and addition of such an unnecessary component increases the manufacturing cost and necessitates the use of a large capacity drive motor.
An object of the invention is thus to provide a dot printer in which all of the above-described drawbacks accompanying a conventional dot printer have been eliminated and which is simple in construction and is capable of operating at a high printing speed.